The present invention relates to continuously operating, particularly heated, gas analysers comprising a mass flow-dependent detector, which must be
(a) operated under low pressure because the measuring gas is not present under sufficient excess pressure and because no suitable temperature-resistant or corrosion-proof pumps may be used upstream of the analyser, and
(b) charged with one or more gas flows apart from with the measuring gas, as this is required for its function.
For this purpose, a suction device is usually positioned at the outlet of the detector which maintains the low pressure constant with respect to atmospheric pressure. The amount of measuring gas which is supplied is adjusted to a specific value by connecting a choke upstream of the detector.
In order to improve the time behaviour, a by-pass is often connected parallel to the detector which contains another choke determining the by-pass flow. However, in this arrangement, a constant mass flow of the measuring gas is only ensured when the pressure of the measuring gas is constant at its inlet.
Continuous flame ionization detectors (FID) and the thermionic detectors based on a similar effect produce a measuring signal which is proportional to the mass flow. For this reason, a constant mass flow of measuring gas has to be metered into the detector. Metering the measuring gas usually involves compressing the measuring gas by means of a pump upstream of the detector to a pressure which is adjusted by a pressure control apparatus to a constant excess pressure with respect to atmospheric pressure and which is connected to the inlet of the detector, operated at atmospheric pressure, via a fixed choke member. In this way, a constant flow of measuring gas is adjusted due to the constant differential pressure at the choke member. This control of the mass flow upstream of the detector has the advantage that it is not influenced by the addition of other gas flows into the detector (which are required for its function, e.g. fuel gas and combustion air for the FID). Any gas compressor which does not change the composition of the measuring gas may be used as the pump. Pumps with mechanically moving parts, e.g. diaphragm pumps, are usually used as the pumps. Overflow controllers (back pressure controllers) are usually used as the pressure control apparatus which allow a variable partial flow of the measuring gas to flow away into the atmosphere through a by-pass line by means of a variable choke. Needle valves, capillaries or nozzles may be used as choke members in the inlet of the detector. The type of measuring gas metering which has been described has proved to be useful in continuous FID devices in the field of emission measurement, in which devices the measuring gas is present as relatively clean air. For uses in the field of emission measurement, this type of metering has considerable disadvantages which derive from the following circumstances with high loads of the measuring gases:
(a) the dew point of the measuring gas may lie considerably above the ambient temperature, and
(b) the measuring gas may have a heavily corrosive effect.
For these reasons, it has proved to be necessary with emission measuring devices to heat all components which come into contact with the measuring gas to temperatures of from 150.degree. to 200.degree. C. However, the disadvantage of heating the measuring gas system to such high temperatures is that the life of some components, such as pumps and pressure controllers, is drastically reduced to a level which is not acceptable.
This can be remedied by using a detector which operates in connection with a low pressure control during suction operation. An apparatus constructed according to this principle is described for example in the article by H. Fischer et al. in GIT March 1974 p. 214-219. The low pressure is controlled in this case by a differential pressure controller which maintains the differential pressure at the outlet of the detector constant with respect to atmospheric pressure. Adjusting the quantity of measuring gas supplied to the detector and thereby adapting the measuring sensitivity to the respective problem to be solved is effected by a capillary which is connected upstream of the detector. A by-pass line bridging the detector which opens out upstream of the capillary and contains another capillary is used to improve the time behaviour of the arrangement. An equalizing reservoir is positioned at the outlet of the detector, the suction device being connected thereto on one side and the differential pressure controller on the other side.
The disadvantage of this apparatus is that it is basically unsuitable for a fluctuating measuring gas pressure. Only the differential pressure between the surrounding air and the equalizing reservoir is maintained constant by the low pressure control. However, pressure fluctuations in the measuring gas and the change in the mass flow of the measuring component, resulting therefrom, cannot be eliminated and result in a systematic error in the reading. The same applies to changes in the flow resistance between the inlet and the detector (e.g. when an inlet filter is soiled).